This invention relates to methods and systems for high speed measuring of targets and, in particular, to methods and systems for high speed measuring of microscopic targets which may be xe2x80x9cnon-cooperative.xe2x80x9d
Recognition of Need
A class of three-dimensional imaging and measurement applications now requires unprecedented demonstration of capability to support new microelectronic and micromechanical fabrication technologies. For example, emerging semiconductor fabrication technologies are directed toward establishing a high density of interconnection between the chip and package. The xe2x80x9cbumped waferxe2x80x9d and miniature ball grid array (xe2x80x9cxcexc-BGAxe2x80x9d) markets are emerging, and large scale growth is predicted. For instance, NEMI (National Electronics Manufacturing Initiative) has clearly indicated that the miniature array technologies are to replace traditional wire bonding interconnects. Manufacturers are experimenting with new processes. Measurement tools to support their efforts will require versatility.
For example, xe2x80x9cdummy wafersxe2x80x9d are used for many experiments, which have a specular and featureless surface onto which interconnects are placed. The appearance is much different than patterned wafers seen in typical production environments. This imaging phenomena is of little concern to the process engineer. In fact, the most difficult imaging problems may coincide with the best choice of process. Industry process development engineers indicate that reflowed spherical solder bumps with a smooth surface finish, sometimes a nearly perfect mirror, may be the preferred technology for the chip interconnects. The surface reflectance will vary because of process engineers"" choices of relative content of lead and tin. Such targets are often xe2x80x9cuncooperative.xe2x80x9d
The chips onto which the balls are placed are subsequently attached to printed circuit boards where both flattened and spherical mating interconnects can be expected, with either a dull or smooth surface finish. All combinations are expected. Other geometric shapes (wire with flat top, cones) can be expected in the future which will pose measurement challenges, particularly when the surface is specular with spherical or cylindrical geometry, including concavities.
Such xe2x80x9cnon-cooperativexe2x80x9d targets, (i.e. those which present challenges for measurement systems as a result of light reflection, scattering, and geometry), are and will continue to be growing in occurrence for semiconductor, micromachining, and mass storage imaging applications. A specific growing need is recognized for an imaging system capable of improving dimensional measurement of xcexc-BGAs and bumped wafers (i.e. xe2x80x9cspherical mirrorsxe2x80x9d on variable wafer backgrounds) and other such targets, which are xe2x80x9cnon-cooperativexe2x80x9d with respect to traditional imaging systems. As inspection and measurement requirements for industries requiring microscopic measurement capabilities, for instance semiconductor and mass storage, become more demanding, extraordinary versatility will be needed for handling wide variation in scale, target geometry, and reflectivity. Similarly, inspection and measurement of circuit boards and the dielectric and conductive materials requires a versatile imaging system, particularly for fine geometries and densely populated component boards.
As mentioned previously, imaging requirements for the semiconductor packaging industry include defect detection as part of Package Visual Inspection (PVI), measurement of xcexc-BGA height, coplanarity, diameter, and wafer defects. High resolution and image clarity obtained from reduction of image artifacts are both required for adequate process characterization. Problems similar to those in the semiconductor area are also present when measuring other miniature parts like micromachined (micromechanical) assemblies, like miniature gears and machines, and components utilized in the mass storage industry, including substrates, disk heads, and flexures.
For example, as illustrated in FIGS. 1 and 7, inspection of a very fine solder bump or ball 20 with a xe2x80x9cpinxe2x80x9d or tip 22 necking down to about 1-3 xcexcm in dimension mounted on a solder pad 24, poses a measurement problem. Manufacturers often examine the tip 22 with an electron microscope for initial evaluation, but such a tool is much too slow for detailed process characterization or real time control.
Also, detection of small xe2x80x9chairlinexe2x80x9d burrs on IC leads is often successful using gray or 3D data using only triangulation, but false alarms are common because background noise and reflection from a container, such as a tray wall 26, can appear similar to the defect such as a burr 27, as illustrated in FIG. 2. Conversely, IC leads 28 of an IC chip 30 may be indistinguishable from the background noise 32. These false alarms are unacceptable and lower yields, thereby decreasing the value of inspection equipment.
xcexc-BGA inspection can be roughly equivalent to measuring a tiny xe2x80x9cspherical mirrorxe2x80x9d (solder ball) mounted on a plane xe2x80x9cmirrorxe2x80x9d (wafer) background; yet, in other cases, where the wafer is patterned and the ball has a lower tin content, is a completely different imaging problem. Solutions to such measurement problems will require versatility for handling the geometric shape and reflectance variation.
Hence, with wafer scale and other sub-micron measurement tasks, the challenges with material properties will grow, not diminish. There is a need to measure substrates, conductors, and thickness of films, or the geometry of micromechanical assemblies such as miniature gears having deep, narrow dimensions and varying optical properties, including partially transparent layers.
Prior Art Technology
Early work on defect detection of features having specular components using camera-based inspection is described in U.S. Pat. No. 5,058,178 and the references cited therein. The method is primarily directed toward lighting and image processing methods for defect detection of bumped wafers. The lighting system included combinations of bright and dark field illumination. Measurement of the diameter can be done with a camera system and appropriate illumination, but accuracy is often limited by light scattering and limited depth of focus when high magnification is required. However, in addition to defect detection and bump presence, there is a need to measure the three dimensional geometry of the bumps for process characterization. The bumps must be coplanar to provide a proper connection, and the diameter within tolerance for a good connection with the bonding pads.
Triangulation is the most commonly used 3D imaging method and offers a good figure of merit for resolution and speed. U.S. Pat. Nos. 5,024,529 and 5,546,189 describe the use of triangulation-based systems for inspection of many industrial parts, including shiny surfaces like pins of a grid array. U.S. Pat. No. 5,617,209 shows an efficient scanning method for grid arrays which has additional benefits for improving accuracy. The method of using an angled beam of radiant energy can be used for triangulation, confocal or general line scan systems. Unfortunately, triangulation systems are not immune to fundamental limitations like occlusion and sensitivity to background reflection. Furthermore, at high magnification, the depth of focus can limit performance of systems, particularly edge location accuracy, when the object has substantial relief and a wide dynamic range (i.e. variation in surface reflectance). In some cases, camera-based systems have been combined with triangulation systems to enhance measurement capability as disclosed in the publication entitled xe2x80x9cAutomatic Inspection of Component Boards Using 3D and Grey Scale Visionxe2x80x9d by D. Svetkoff et al., PROCEEDINGS INTERNATIONAL SYMPOSIUM ON MICROELECTRONICS, 1986.
Confocal imaging, as originally disclosed by Minsky in U.S. Pat. No. 3,013,467, and publications: (1) xe2x80x9cDynamic Focusing in the Confocal Scanning Microscopexe2x80x9d by T. Wilson et al.; (2) xe2x80x9cDigital Image Processing of Confocal Imagesxe2x80x9d by I. J. Cox and C. J. R. Sheppard; (3) xe2x80x9cThree-Dimensional Surface Measurement Using the Confocal Sensing Microscopexe2x80x9d by D. K. Hamilton and T. Wilson; (4) xe2x80x9cScanning Optical Microscope Incorporating a Digital Framestore and Microcomputerxe2x80x9d by I. J. Cox and C. J. R. Sheppard; and (5) xe2x80x9cDepth of Field in the Scanning Microscopexe2x80x9d by C. J. R. Sheppard and T. Wilson, is similar to computerized tomography where slices in depth are sequentially acquired and the data is used to xe2x80x9creconstructxe2x80x9d a light scattering volume. In principle, an image is always formed of an object at a focal plane as taught in elementary physics, but over a region of depth there are an infinite number of planes which are out of focus yet return energy. That is to say that the lens equation for image formation is based on an idealization of an xe2x80x9cobject planexe2x80x9d and xe2x80x9cimage planexe2x80x9d.
In the case of conventional confocal imaging, the slices are determined from the in-focus plane, and out-of-focus light (in front and back of the focal plane) is strongly attenuated with a pinhole or slit. Typical confocal systems use fine increments for axial positioning for best discrimination between adjacent layers in depth, for example, semi-transparent biological samples. However, the method need not be restricted to the traditional transparent or translucent objects, but can be applied both as a depth measurement tool and image enhancement method using reflected light for contrast improvement through stray light rejection. As with any method, there are advantages and disadvantages.
Application of confocal imaging to semiconductor measurement is disclosed in U.S. Pat. Nos. 4,689,491, 5,479,252 and 5,248,876. Operation of several confocal systems is described in U.S. Pat. Nos. 4,827,125; 4,863,226; 4,893,008; 5,153,428; 5,381,236; 5,510,894; 5,594,235; and 5,483,055 and H 1,530. Much of the recent work is directed toward improvements, resulting in reduction of the image memory storage requirements (store maximum, not volume), improving the efficiency and fine positioning capability of autofocus systems (coarse/fine search), exposure control for improved dynamic range, and some image enhancement methods.
Similarly, variations in confocal acquisition methods are taught in the art to solve specific problems or optimize designs for specific applications as taught in U.S. Pat. Nos. 5,239,178 and 4,873,653. However, present confocal systems are constrained by sequential slicing of the volume, whereas triangulation systems detect the top surface of the volume (profile) directly resulting in much higher speed.
In U.S. Pat. No. 5,448,359 such speed limitations are partially circumvented by utilizing a plurality of detectors and spatial filters in the confocal receiver optical path. A circuit to locate the detector producing maximum intensity is disclosed.
Similarly, USSR patent document No. 868,341 discloses a plurality of detectors with apertures (confocal) and electronic circuitry to obtain focus (3D) information about objects. The intensity of each detector is compared and used to adjust the position of the imaging system along the optical axis so as to clear the mismatch. In each case, a tradeoff is determined between depth sensitivity, complexity, and measurement speed.
Other approaches to imaging of xe2x80x9cnon-cooperativexe2x80x9d targets, many directed toward solder joint inspection, have been proposed to measure depth or fillet shape. These are described in the Chen et al. U.S. Pat. No. 5,118,192 and a Nagoya solder joint inspection system described in xe2x80x9cNLB Laser Inspectorxe2x80x94NLB-7700M Specificationsxe2x80x9d by Nagoya Electric Works Co., Ltd. 1994. The system uses specularly reflected light to examine the shape of solder fillets, and to determine presence/absence of solder. FIG. E in Section 6 thereof shows a missing fillet and the signals received from a plurality of detectors. A detector 6 corresponds to an xe2x80x9con-axisxe2x80x9d detector, and the information is useful for estimating the diameter of the solder bump. For instance, the detector 6 receives a large signal near the top of the ball, a weak signal from the curved edge, and typically a strong signal from the area adjacent to the bump. However, narrow angle multiple reflections from the edge of the ball can corrupt the measurement and result in ambiguous edge locations. Furthermore, the sensitivity of the system may not be adequate to determine the height of regions which do not have a substantial specular reflection component.
Similarly, a recent version of the IPK solder joint inspection system manufactured by Panasert includes a coaxial detector with a triangulation-based sensing system as illustrated in their brochure entitled xe2x80x9cIPK-Vxe2x80x9d believed to be published in 1997. The xcexc-BGA, bumped die, and numerous other problems range from scenarios where prior art technology is adequate, but in many cases unacceptable, and even inoperable conditions exist.
Wafer measurement and defect detection systems have utilized multiple detectors advantageously. U.S. Pat. No. 5,416,594 describes a system which uses both reflected and scattered light for detection of defects and thin film measurements. The reflected beam is received at an angle of reflection which is non-collinear with the transmitted beam and the scattered light is collected over a relatively large angle which excludes the reflected beam energy. The scattered light beam, representative of surface defects, may be collected at an angle which is widely separated (more than 30 deg.) from the incident beam. The off-axis illumination and the corresponding reflected beam are utilized for film thickness measurements, sometimes with multiple laser wavelengths. The scattered light signal is analyzed in conjunction with that representing the reflected light. Although the imaging geometry is well matched to the specific cited inspection requirements, there are several potential disadvantages encountered when attempting to simultaneously provide information about surface defects and say, the peak height of interconnects like solder bumps (which have substantial height) and the corresponding diameter and shape.
Commercial success has not been widespread, although many approaches have been proposed. Hence, there is a need for a system and method for three-dimensional imaging capable of performing with both xe2x80x9ccooperativexe2x80x9d and xe2x80x9cnon-cooperativexe2x80x9d targets. To be useful, the method and system must be accurate, robust, and have high measurement speed, the latter being a traditional limit to the use of widespread confocal imaging.
A method of the present invention overcomes the limitations of the prior art imaging of non-cooperative targets by illuminating a surface with a scanning beam, acquiring data from at least one triangulation-based channel, and acquiring in parallel or sequentially at least one slice of confocal image data having substantially perfect temporal and spatial registration with the triangulation-based sensor data, allowing for fusion or processing of the data for use with a predetermined measurement algorithm.
The objects of a system of the present invention are met by utilizing a combination of confocal and triangulation-based data acquisition, with a control algorithm guiding the cooperative data acquisition and subsequent processing.
The invention is a method and system for developing three-dimensional information about an object by illuminating an object with a focused beam of electromagnetic radiation incident from a first direction. A detector of electromagnetic radiation is placed at a first location for receiving reflected radiation which is substantially optically collinear with the incident beam of electromagnetic radiation, and the detection system includes a spatial filter for attenuating background energy. Another detector of electromagnetic radiation is placed at a second location which is non-collinear with respect to the incident beam. The detector has a position sensitive axis. Digital data is derived from signals produced by said first and second detectors. The digital data is then processed to generate information about the object.
Specific objects of the invention include:
An object of the invention is to provide an integrated method and system for high speed measuring to obtain measurements for conductor traces (height xcx9c1-3 xcexcm) and/or interconnects (i.e. 10-300 xcexcm bumps) on semiconductor devices.
An object of the invention is to provide a method and system for high speed measuring which has diverse measurement and defect detection capability with a combination of a confocal sensor and triangulation allowing for measurement of miniature, complex geometry present in the microelectronics, micromechanical, and disk storage industries.
An object of the invention is to provide a method and system for high speed measuring to obtain information from either of two channels used to guide subsequent data acquisition operations in either or both channels. For example, sparse data may be acquired with a triangulation-based system at high speed, and the information used to guide the high speed selection of confocal slices, perhaps in windowed regions. FIG. 3 illustrates imaging geometry of a solder ball 29 (i.e. spherical mirror) of radius R (i.e. R less than 150 xcexcm typically) formed on a pad 31.
An object of the invention is to provide a high speed method and system for measuring which can obtain reasonable height estimates of the bumps or xe2x80x9cspherical mirrorsxe2x80x9d in a xe2x80x9cpre-screeningxe2x80x9d operation and locate defective bumps or wafers at high speed. The results would define the range for additional slices (i.e. if needed) for precise verification of the geometry of regions passing the xe2x80x9cpre-screeningxe2x80x9d test. Therefore, maintaining wafer inspection times will remain as minutes, not hours. For xe2x80x9csparsexe2x80x9d patterns, xe2x80x9cwindowingxe2x80x9d could increase the speed of measurement for localized regions. FIGS. 4a and 4b are top schematic images of specular solder balls 34 (indicated by phantom lines in FIG. 4a) using triangulation and confocal imaging, respectively, in accordance with the present invention. The balls 34 of FIG. 4b have specular ball tips 35 formed on pads 36 which, in turn, are located on a shiny xe2x80x9cdummyxe2x80x9d wafer 38. The 3D image of FIG. 4a (i.e. including specular reflections from regions 35xe2x80x2 of the ball 34 adjacent the ball tips) is formed by a triangulation-based system having dual detectors to provide Z measurement, bump presence and defect information. The confocal slice image of FIG. 4b provides diameter, Z measurement and defect information. In both FIGS. 4a and 4b, a flat bump having diffuse reflection is indicated at 40, an empty pad (i.e. missing bump) is indicated at 42, and a smashed bump (i.e. defect) is indicated at 44.
Referring specifically to FIG. 5, an object of the invention is to provide a high speed method and system for measuring a miniature spherical mirror like a solder ball 46 or wafer, mounted on a plane mirror or pad 48 formed on a substrate 50 and producing a very high contrast bump-background image allowing for accurate measurement of diameter, devoid of occlusion and with minimal reflection noise for many pad backgrounds. FIG. 5 shows a spatial filter 52 through which an incident ray 54 passes and bounces off the ball surface to form reflected rays 56, multiple reflections 58, and specular reflection 60. The spatial filter 52 (i.e. confocal slit) provides the indicated filtering action.
An object of the invention is to provide a high speed method and system for measuring which have significant advantages over conventional camera and lighting systems, even with relatively few slices of spatially filtered data.
An object of the invention is to provide a high speed method and system for measuring to, in turn, provide gray scale contrast improvement of the image of FIG. 2 for possible detection of defects and reduction of false xe2x80x9cacceptsxe2x80x9d and xe2x80x9crejectsxe2x80x9d (i.e. xe2x80x9cerrorxe2x80x9d region 33) in any number of applications through stray light rejection. One such classification of burrs 27 and similar defects, like those specified for electronic Package Visual Inspection (PVI), may be satisfied with this method and system and would otherwise be difficult. FIG. 6 is a confocal slice of the IC chip 30 of FIG. 2 located in the tray 26 of FIG. 2. FIG. 6 illustrates the effect of spatial filtering with best focus near the nominal pad and burr locations. With the present invention, the data of FIG. 6 is combined with 3D triangulation data for improved classification. Also, visualization and measurement of small bumps and pits could be improved. Furthermore, discrimination of edges which is difficult in the presence of multiple reflection is provided herein.
An object of the invention is to provide a high speed method and system for measuring to, in turn, overcome limits of triangulation-based imaging for xe2x80x9cmirroredxe2x80x9d wafer backgrounds, where triangulation often requires photon limited detection or nearly so, and to provide a focus-based depth measurement method and system which operates at high speed.
An object of the invention is to provide a method and system for measuring at high speed for measurement of ball bumps, and rigid wire interconnects within the semiconductor industry.
Referring again to FIGS. 1 and 7, an object of the invention is to provide a method and system for high speed measuring of objects having complex geometry, for instance xe2x80x9cball bumpsxe2x80x9d 20 and rigid wires 22, with the speed advantages of a triangulation/confocal combination while overcoming xe2x80x9cenclosed energyxe2x80x9d limitations and resulting corruption of the xe2x80x9csignalxe2x80x9d by optical noise from reflection of the sidelobe energy to the background (as illustrated in FIG. 7), producing false readings in triangulation-based systems. In this case, a confocal channel produces a higher optical signal-to-noise and background rejection, while a triangulation-based system rapidly measures the other features, albeit at least two passes might be required because of the pin height relative to the necessarily restricted depth of focus.
An object of the invention is to provide an integrated method and system for high speed measuring having substantially perfect temporal and spatial registration between two sensors or subsystems of the system which allows xe2x80x9cfusionxe2x80x9d of the data, with selection of the best sensor data based upon reflectance and contrast, perhaps on a pixel-by-pixel basis.
An object of the invention is to provide a versatile method and system for high speed measuring of targets on the wafer scale for inspection and measurement. At such higher magnification, material properties vary greatly, from translucent to opaque, and xe2x80x9cmirror-likexe2x80x9d to matte.
An object of the invention is to provide a method and system for high speed measuring to provide improved discrimination of metallic surfaces from the translucent backgrounds, and to measure materials such as conductive epoxy used for interconnects. Some applications in the optical storage industry may be best solved with this type of technology (flexure measurement) and at higher magnification (high contrast, disk head measurement).
An object of the invention is to provide a method and system for high speed measuring which introduces a feature of increased gray scale contrast and fidelity from the region of the beam waist, through at least rudimentary xe2x80x9cdepth-through-focus detectionxe2x80x9d capability. At very high magnification, a confocal channel either xe2x80x9ccompetesxe2x80x9d or xe2x80x9ccooperatesxe2x80x9d with dual-detector triangulation for the best imaging mode.
An object of the invention is to provide an integrated method and system for high speed measuring which can include both high N.A. (numerical aperture) optics and lower N.A. for use with either confocal or triangulation channels realized with wavelength, time or spatial multiplexing methods, as illustrated in FIG. 8.
An object of the invention is to provide a method and system for high speed measuring which provides selectable lateral and depth resolution for confocal and triangulation-based imaging through the use of multiplexing and programmable or selectable height resolution.
An object of the invention is to provide a method and system for high speed measuring including high grey scale resolution and dynamic range (sufficient to avoid automatic gain or light control requirements) and processing with smoothing algorithms. The smoothing algorithms may be adapted to include known information regarding the physical characteristics of the object.
An object of the present invention is to provide a method and system for high speed measuring by obtaining confocal and/or triangulation data rapidly. Objects which are reflective, such as solder joints, substrates and wafers are substantially opaque in a homogeneous medium such as air, unlike several objects traditionally xe2x80x9cslicedxe2x80x9d with the confocal technique. As such, an object of the invention is to estimate the depth of reflective objects using estimation techniques and relatively few slices compared to traditional xe2x80x9cpeak detectionxe2x80x9d systems utilized for confocal imaging. The smoothing and estimation techniques could be utilized with a single confocal detector when data is acquired with axial translation, with multiple detectors involving no translation or a combination of the two.
A further object of the present invention is to provide a method and system for high speed measuring by adapting smoothing and/or estimation algorithms based upon a priori information regarding the physical characteristics of objects within a region of interest, thereby avoiding corruption of the measurements associated with peak search methods.
A further object of the invention is to provide measurement capability of both xe2x80x9cfeaturelessxe2x80x9d and textured surfaces using an appropriate selection of information.
An object of the invention is to provide an improved method of measuring using confocal imaging, used alone or in combination with triangulation, where acquisition times are reduced with the use of a solid state beam deflector having retrace times on the order of 1-10 microseconds, whereby pixel rates well in excess of video rates are achievable.
A further object of the invention is to provide an improved method of measuring using confocal imaging where mechanical motion requirements for axial translation of the position of focus of the illumination beam is reduced or eliminated.
In carrying out the above objects and other objects of the present invention, a method is provided for developing dimensional information about an object on a specular background utilizing a scanning system having a sensor. The scanning system scans an illumination beam of electromagnetic energy. The method includes the step of determining reference data based on an illumination beam reflected from the specular background. The method further includes the step of positioning the sensor based on the reference data so that a waist of the illumination beam substantially coincides with an expected predetermined 3D location of the object so as to enhance contrast and obtain three-dimensional sensor data and/or confocal sensor data. The method finally includes the step of processing the sensor data to obtain the dimensional information.
Still further in carrying out the above objects and other objects of the present invention, a system is provided for developing dimensional information about an object. The confocal system includes at least one illuminator for illuminating the object with at least one beam of electromagnetic energy to obtain at least one reflected beam of electromagnetic energy, a confocal detector for detecting the at least one reflected beam of electromagnetic energy and producing at least one signal, a signal processor for processing the at least one signal to obtain confocal data and a data processor having digital data processing data smoothing and curve fitting algorithms for processing the confocal data with a priori knowledge about the object to obtain the dimensional information whereby the accuracy of the confocal data is improved (i.e., particularly with the use of fewer slices acquired at relatively coarse increments with respect to the attainable height resolution).
Further in carrying out the above objects and other objects of the present invention, a method is provided for inspecting bumps on a wafer. The method includes the steps of acquiring reference data based on 3D information obtained from either a confocal subsystem or a triangulation subsystem having a triangulation sensor. The method further includes the step of generating a scan based upon the reference data to obtain 3D data wherein the 3D data is obtained from the triangulation sensor. The method finally includes the step of determining height of the bumps based on the 3D data.
Yet still further in carrying out the above objects and other objects of the present invention, a method is provided for developing dimensional information about an array of objects, each of the objects including a surface. The method includes the steps of obtaining a first set of data representing maximum specular reflections from the surfaces of the objects, computing height estimate data for the array of objects utilizing the first set of data and analyzing the height estimate data to obtain an estimate of the height.
In further carrying out the above objects and other objects of the present invention, a method is provided for measuring at least one dimension of an interconnect on a specular wafer. The method includes the step of measuring the wafer at three or more non-colinear locations to obtain reference data. The method includes further includes forming a reference plane from the reference data. The method also includes the step of scanning the wafer to obtain scan data based on the reference plane. The method finally includes the step of determining the at least one dimension of the interconnect based on the scan data.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.